Hi, @uberwaltz,
I don't have a definitive answer for you right now because all of the testing/try-outs I've done with one/two filters of different sizes and make-up. Add to that, my pre-steaming really helps prolong water/filter changes.
If I had to guess, and it's probably subjective, once every 6 months would be a good guess. That's cleaning an average of 60 records a month.
I think people who've never US cleaned before would be very surprised at how much junk comes off their "clean" lps.